Re: [PATCH v4 18/21] fuse: Add support for pid namespaces

On Wed, Jul 20, 2016 at 2:52 PM, Seth Forshee
[off-list ref] wrote:

It sounds like we need to remove the restriction on accessing the
filesystem from a different pid namespace. I don't think this poses a
security problem. However there's no pid mapping that is usable by the
userspace fuse process, so what do we put in the fuse request? Probably
the only candidates are 0 and 0xffffffff.

So a question for the fuse developers - is one value or the other
preferrable for fuse_in_header.pid when the pid cannot be mapped, and is
this going to cause problems for any fuse filesystems? I suspect that
few filesystems actually look at the pid anyway, and already for a
filesystem mounted in a pid namespace the values being given to
userspace won't be correct for the namespace of the fuse process.

pid = 0 sounds good.

The pid from the request is used for example to get the auxiliary
group list by libfuse (fuse_req_getgroups()).   That's not used by all
filesystems and it will return an error in case it can't find the proc
entry (which it won't for pid == 0).

It would be nice if we could transfer the group list through the
userspace/kernel protocol, since then it wouldn't depend on proc and
on being in the same pid namespace.  But that's another story.

Thanks,
Miklos